Background
SAM and HD domain-containing deoxynucleoside triphosphate triphosphohydrolase protein 1 (SAMHD1) is an intracellular protein that regulates a stable deoxynucleoside triphosphates (dNTPs) for normal cellular function. Many studies have documented SAMHD1-mediated restriction of RNA retroviruses and different DNA viruses in non-dividing myeloid cells, where it acts to deplete cellular dNTP pools to inhibit replication of the viral genome. Less is currently known regarding its ability to restrict the replication of other RNA viruses in different cell types. In this study, we investigate the role of SAMHD1 in modulating the replication of RNA viruses associated with respiratory virus infection, with a particular focus on seasonal influenza A virus (IAV). Moreover, our studies have assessed the ability of SAMHD1 to modulate IAV replication in two key cell types associated with the pathogenesis of IAV, namely epithelial and macrophage-like cells.

Methods
qPCR was used to examine expression of endogenous SAMHD1 in response to IAV infection and/or treatment with recombinant type I interferon (IFN) in epithelial (A549) and macrophage-like (THP-1) cells. CRISPR/Cas9 knockout (KO) of endogenous SAMHD1 was performed to assess the role of SAMHD1 in modulating IAV replication in A549 cells or in differentiated (d)THP-1. IAV infection and replication were also assessed in A549 and dTHP-1 SAMHD1 KO cells with stable doxycycline (DOX)-induced expression of SAMHD1 or of phosphorylation mutants of SAMHD1.

Results
CRISPR/Cas9 KO of endogenous SAMHD1 in dTHP-1 or A549 cells did not enhance virus release following infection with seasonal IAV. Moreover, reconstitution of KO cells with DOX-inducible SAMHD1 did not restrict the growth of seasonal IAV in dTHP-1 or A549 cells, despite SAMHD1-mediated restriction of a DNA virus (HSV-1) in both cell types. In contrast to seasonal IAV, we also observed restriction of highly pathogenic avian influenza (HPAI) A(H5N1) IAV replication in dTHP-1 cells expressing DOX-inducible SAMHD1.

Conclusions
Together, these studies indicate that neither endogenous nor DOX-inducible SAMHD1 restricts the growth of seasonal IAV in macrophage-like or epithelial cells.